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<tr class="t-nv-h2"><td colspan="5"> 标号 </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="statements.html" title="cpp/language/statements"> <span class="t-spar">label</span> : <span class="t-spar">statement</span></a></td></tr>
<tr class="t-nv-h2"><td colspan="5"> 表达式语句 </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="statements.html" title="cpp/language/statements"> <span class="t-spar">expression</span> ; </a></td></tr>
<tr class="t-nv-h2"><td colspan="5"> 复合语句 </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="statements.html" title="cpp/language/statements"> { <span class="t-spar">statement</span>... } </a></td></tr>
<tr class="t-nv-h2"><td colspan="5"> 选择语句 </td></tr>
<tr class="t-nv"><td colspan="5"> <strong class="selflink"> if</strong></td></tr>
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<tr class="t-nv"><td colspan="5"> <a href="goto.html" title="cpp/language/goto"> goto</a></td></tr>
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</div><div></div></div></div></div><div class="t-navbar-sep"> </div></div>
<p>有条件地执行另一条语句。
</p><p>用于需要基于运行时或编译时条件执行的代码。
</p>
<h3><span class="mw-headline" id=".E8.AF.AD.E6.B3.95">语法</span></h3>
<table class="t-sdsc-begin">

<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">attr</span><span class="t-mark">(可选)</span> <code><b>if</b></code> <code><b>(</b></code> <span class="t-spar">条件</span> <code><b>)</b></code> <span class="t-spar">true分支语句</span>
</td>
<td> (1)
</td>
<td> <span class="t-mark-rev t-until-cxx17">(C++17 前)</span>
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">attr</span><span class="t-mark">(可选)</span> <code><b>if</b></code> <code><b>constexpr</b></code><span class="t-mark">(可选)</span> <code><b>(</b></code> <span class="t-spar">初始化语句</span><span class="t-mark">(可选)</span> <span class="t-spar">条件</span> <code><b>)</b></code> <span class="t-spar">true分支语句</span>
</td>
<td> (1)
</td>
<td> <span class="t-mark-rev t-since-cxx17">(C++17 起)</span>
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">attr</span><span class="t-mark">(可选)</span> <code><b>if</b></code> <code><b>(</b></code> <span class="t-spar">条件</span> <code><b>)</b></code> <span class="t-spar">true分支语句</span> <code><b>else</b></code> <span class="t-spar">false分支语句</span>
</td>
<td> (2)
</td>
<td> <span class="t-mark-rev t-until-cxx17">(C++17 前)</span>
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">attr</span><span class="t-mark">(可选)</span> <code><b>if</b></code> <code><b>constexpr</b></code><span class="t-mark">(可选)</span> <code><b>(</b></code> <span class="t-spar">初始化语句</span><span class="t-mark">(可选)</span> <span class="t-spar">条件</span> <code><b>)</b></code> <span class="t-spar">true分支语句</span> <code><b>else</b></code> <span class="t-spar">false分支语句</span>
</td>
<td> (2)
</td>
<td> <span class="t-mark-rev t-since-cxx17">(C++17 起)</span>
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr></table>
<table class="t-par-begin">


<tr class="t-par">
<td>  <span class="t-spar">attr</span><span class="t-mark-rev t-since-cxx11">(C++11)</span>
</td>
<td> -
</td>
<td>  任意数量的<a href="attributes.html" title="cpp/language/attributes">属性</a>
</td></tr>
<tr class="t-par">
<td>  <span class="t-spar">条件</span>
</td>
<td> -
</td>
<td>  下列之一
<ul><li> <a href="implicit_cast.html" title="cpp/language/implicit conversion">按语境转换</a>为 bool 的<a href="expressions.html" title="cpp/language/expressions">表达式</a>
</li><li> 单个非数组变量的带花括号或等号<a href="initialization.html" title="cpp/language/initialization">初始化器</a>的<a href="declarations.html" title="cpp/language/declarations">声明</a>。
</li></ul>
</td></tr>
<tr class="t-par">
<td>  <span class="t-spar">初始化语句</span><span class="t-mark-rev t-since-cxx17">(C++17)</span>
</td>
<td> -
</td>
<td>  下列之一
<ul><li> 一条<a href="statements.html" title="cpp/language/statements">表达式语句</a>（可以是<i>空语句</i>“<code><b>;</b></code>”）
</li><li> 一条<a href="declarations.html" title="cpp/language/declarations">简单声明</a>，典型的是带初始化器的变量声明，但它可以声明任意多变量，或是一条分解声明
</li></ul>
<dl><dd>注意，任何 <span class="t-spar">初始化语句</span> 必然以分号 <code><b>;</b></code> 结束，这是它常被非正式地描述成后随分号的表达式或声明的原因。
</dd></dl>
</td></tr>
<tr class="t-par">
<td>  <span class="t-spar">true分支语句</span>
</td>
<td> -
</td>
<td>  任何<a href="statements.html" title="cpp/language/statements">语句</a>（常为复合语句），当 <span class="t-spar">条件</span> 求值为 <code>true</code> 时执行
</td></tr>
<tr class="t-par">
<td>  <span class="t-spar">false分支语句</span>
</td>
<td> -
</td>
<td>  任何<a href="statements.html" title="cpp/language/statements">语句</a>（常为复合语句），当 <span class="t-spar">条件</span> 求值为 <code>false</code> 时执行
</td></tr></table>
<h3><span class="mw-headline" id=".E8.A7.A3.E9.87.8A">解释</span></h3>
<p>若 <span class="t-spar">条件</span> 在转换到 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span></span></span> 后产生 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span>，则执行 <span class="t-spar">true分支语句</span>。
</p><p>若 if 语句的 else 部分存在，且 <span class="t-spar">条件</span> 在转换到 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span></span></span> 后产生 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span>，则执行 <span class="t-spar">false分支语句</span>。
</p><p>在 if 语句的第二形式（包含 else）中，若 <span class="t-spar">true分支语句</span> 亦是 if 语句，则内层 if 语句必须也含有 else 部分（换言之，嵌套 if 语句中，else 关联到最近的尚未有 else 的 if）。
</p>
<div class="t-example"><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">运行此代码</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;iostream&gt;</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
    <span class="co1">// 带 else 子句的简单 if 语句</span>
    <span class="kw4">int</span> i <span class="sy1">=</span> <span class="nu0">2</span><span class="sy4">;</span>
    <span class="kw1">if</span> <span class="br0">(</span>i <span class="sy1">&gt;</span> <span class="nu0">2</span><span class="br0">)</span> <span class="br0">{</span>
        <a href="../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> i <span class="sy1">&lt;&lt;</span> <span class="st0">" 大于 2<span class="es1">\n</span>"</span><span class="sy4">;</span>
    <span class="br0">}</span> <span class="kw1">else</span> <span class="br0">{</span>
        <a href="../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> i <span class="sy1">&lt;&lt;</span> <span class="st0">" 不大于 2<span class="es1">\n</span>"</span><span class="sy4">;</span>
    <span class="br0">}</span>
 
    <span class="co1">// 嵌套 if 语句</span>
    <span class="kw4">int</span> j <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span>
    <span class="kw1">if</span> <span class="br0">(</span>i <span class="sy1">&gt;</span> <span class="nu0">1</span><span class="br0">)</span>
        <span class="kw1">if</span> <span class="br0">(</span>j <span class="sy1">&gt;</span> <span class="nu0">2</span><span class="br0">)</span>
            <a href="../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> i <span class="sy1">&lt;&lt;</span> <span class="st0">" &gt; 1 且 "</span> <span class="sy1">&lt;&lt;</span> j <span class="sy1">&lt;&lt;</span> <span class="st0">" &gt; 2<span class="es1">\n</span>"</span><span class="sy4">;</span>
        <span class="kw1">else</span> <span class="co1">// 此 else 属于 if (j &gt; 2), 而不是 if (i &gt; 1)</span>
            <a href="../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> i <span class="sy1">&lt;&lt;</span> <span class="st0">" &gt; 1 且 "</span> <span class="sy1">&lt;&lt;</span> j <span class="sy1">&lt;&lt;</span> <span class="st0">" &lt;= 2<span class="es1">\n</span>"</span><span class="sy4">;</span>
 
   <span class="co1">// 以下声明可用于含 dynamic_cast 的条件</span>
   <span class="kw1">struct</span> Base <span class="br0">{</span>
        <span class="kw1">virtual</span> ~Base<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>
   <span class="br0">}</span><span class="sy4">;</span>
   <span class="kw1">struct</span> Derived <span class="sy4">:</span> Base <span class="br0">{</span>
       <span class="kw4">void</span> df<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <a href="../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"df()<span class="es1">\n</span>"</span><span class="sy4">;</span> <span class="br0">}</span>
   <span class="br0">}</span><span class="sy4">;</span>
   Base<span class="sy2">*</span> bp1 <span class="sy1">=</span> new Base<span class="sy4">;</span>
   Base<span class="sy2">*</span> bp2 <span class="sy1">=</span> new Derived<span class="sy4">;</span>
 
   <span class="kw1">if</span> <span class="br0">(</span>Derived<span class="sy2">*</span> p <span class="sy1">=</span> <span class="kw1">dynamic_cast</span><span class="sy1">&lt;</span>Derived<span class="sy2">*</span><span class="sy1">&gt;</span><span class="br0">(</span>bp1<span class="br0">)</span><span class="br0">)</span> <span class="co1">// 转型失败，返回 nullptr</span>
       p<span class="sy2">-</span><span class="sy1">&gt;</span>df<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>  <span class="co1">// 不执行</span>
 
   <span class="kw1">if</span> <span class="br0">(</span><span class="kw4">auto</span> p <span class="sy1">=</span> <span class="kw1">dynamic_cast</span><span class="sy1">&lt;</span>Derived<span class="sy2">*</span><span class="sy1">&gt;</span><span class="br0">(</span>bp2<span class="br0">)</span><span class="br0">)</span> <span class="co1">// 转型成功</span>
       p<span class="sy2">-</span><span class="sy1">&gt;</span>df<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>  <span class="co1">// 执行</span>
<span class="br0">}</span></pre></div></div>
<p>输出：
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="text source-text"><pre class="de1">2 不大于 2
2 &gt; 1 且 1 &lt;= 2
df()</pre></div></div> 
</div>
<p><br>
</p>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx17"><td>
<h4> <span class="mw-headline" id=".E5.B8.A6.E5.88.9D.E5.A7.8B.E5.8C.96.E5.99.A8.E7.9A.84_if_.E8.AF.AD.E5.8F.A5">带初始化器的 if 语句</span></h4>
<p>若使用 <span class="t-spar">初始化语句</span>，则 if 语句等价于
</p>
<table class="t-sdsc-begin">

<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td class="t-sdsc-nopad"> <code><b>{</b></code><br>
<dl><dd><span class="t-spar">初始化语句</span> <br>
</dd><dd><code><b>if</b></code> <code><b>constexpr</b></code><span class="t-mark">(可选)</span> <code><b>(</b></code> <span class="t-spar">条件</span> <code><b>)</b></code>  <br>
<dl><dd><span class="t-spar">true分支语句</span> <br>
</dd></dl>
</dd></dl>
<p><code><b>} </b></code>
</p>
</td>
<td class="t-sdsc-nopad">
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr></table>
<p>或
</p>
<table class="t-sdsc-begin">

<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td class="t-sdsc-nopad"> <code><b>{</b></code><br>
<dl><dd><span class="t-spar">初始化语句</span> <br>
</dd><dd><code><b>if</b></code> <code><b>constexpr</b></code><span class="t-mark">(可选)</span> <code><b>(</b></code> <span class="t-spar">条件</span> <code><b>)</b></code>  <br>
<dl><dd><span class="t-spar">true分支语句</span> <br>
</dd></dl>
</dd><dd><code><b>else</b></code>
<dl><dd><span class="t-spar">false分支语句</span> <br>
</dd></dl>
</dd></dl>
<p><code><b>} </b></code>
</p>
</td>
<td class="t-sdsc-nopad">
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr></table>
<p>但 <span class="t-spar">初始化语句</span> 所声明的名字（若 <span class="t-spar">初始化语句</span> 是声明）和 <span class="t-spar">条件</span> 所声明的名字（若 <span class="t-spar">条件</span> 是声明）处于同一作用域中，同时也是两条 <span class="t-spar">语句</span> 所在的作用域。
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><a href="../container/map.html"><span class="kw1281">std::<span class="me2">map</span></span></a><span class="sy1">&lt;</span><span class="kw4">int</span>, <a href="../string/basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a><span class="sy1">&gt;</span> m<span class="sy4">;</span>
<a href="../thread/mutex.html"><span class="kw2154">std::<span class="me2">mutex</span></span></a> mx<span class="sy4">;</span>
<span class="kw4">extern</span> <span class="kw4">bool</span> shared_flag<span class="sy4">;</span> <span class="co1">// 由 mx 保证</span>
<span class="kw4">int</span> demo<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
   <span class="kw1">if</span> <span class="br0">(</span><span class="kw4">auto</span> it <span class="sy1">=</span> m.<span class="me1">find</span><span class="br0">(</span><span class="nu0">10</span><span class="br0">)</span><span class="sy4">;</span> it <span class="sy3">!</span><span class="sy1">=</span> m.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> it<span class="sy2">-</span><span class="sy1">&gt;</span>second.<span class="me1">size</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span>
   <span class="kw1">if</span> <span class="br0">(</span><span class="kw4">char</span> buf<span class="br0">[</span><span class="nu0">10</span><span class="br0">]</span><span class="sy4">;</span> <a href="../io/c/fgets.html"><span class="kw1823">std::<span class="me2">fgets</span></span></a><span class="br0">(</span>buf, <span class="nu0">10</span>, <span class="kw2">stdin</span><span class="br0">)</span><span class="br0">)</span> <span class="br0">{</span> m<span class="br0">[</span><span class="nu0">0</span><span class="br0">]</span> <span class="sy2">+</span><span class="sy1">=</span> buf<span class="sy4">;</span> <span class="br0">}</span>
   <span class="kw1">if</span> <span class="br0">(</span><a href="../thread/lock_guard.html"><span class="kw2166">std::<span class="me2">lock_guard</span></span></a> lock<span class="br0">(</span>mx<span class="br0">)</span><span class="sy4">;</span> shared_flag<span class="br0">)</span> <span class="br0">{</span> unsafe_ping<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> shared_flag <span class="sy1">=</span> <span class="kw2">false</span><span class="sy4">;</span> <span class="br0">}</span>
   <span class="kw1">if</span> <span class="br0">(</span><span class="kw4">int</span> s<span class="sy4">;</span> <span class="kw4">int</span> count <span class="sy1">=</span> ReadBytesWithSignal<span class="br0">(</span><span class="sy3">&amp;</span>s<span class="br0">)</span><span class="br0">)</span> <span class="br0">{</span> publish<span class="br0">(</span>count<span class="br0">)</span><span class="sy4">;</span> raise<span class="br0">(</span>s<span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span>
   <span class="kw1">if</span> <span class="br0">(</span><span class="kw4">auto</span> keywords <span class="sy1">=</span> <span class="br0">{</span><span class="st0">"if"</span>, <span class="st0">"for"</span>, <span class="st0">"while"</span><span class="br0">}</span><span class="sy4">;</span>
       <a href="../algorithm/all_any_none_of.html"><span class="kw1566">std::<span class="me2">any_of</span></span></a><span class="br0">(</span>keywords.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span>, keywords.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span>,
                   <span class="br0">[</span><span class="sy3">&amp;</span>s<span class="br0">]</span><span class="br0">(</span><span class="kw4">const</span> <span class="kw4">char</span><span class="sy2">*</span> kw<span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> s <span class="sy1">==</span> kw<span class="sy4">;</span> <span class="br0">}</span><span class="br0">)</span><span class="br0">)</span> <span class="br0">{</span>
     <a href="../io/cerr.html"><span class="kw1760">std::<span class="me2">cerr</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"Token 不能是关键词<span class="es1">\n</span>"</span><span class="br0">)</span><span class="sy4">;</span>
   <span class="br0">}</span>
<span class="br0">}</span></pre></div></div>
</td>
<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td></tr>
</table>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx17"><td>
<h4> <span class="mw-headline" id="constexpr_if">constexpr if</span></h4>
<p>以 <code><b>if constexpr</b></code> 开始的语句被称为<i>constexpr if 语句</i>。
</p><p>在 constexpr if 语句中，<span class="t-spar">条件</span> 的值必须是<a href="constant_expression.html#.E7.BB.8F.E8.BD.AC.E6.8D.A2.E7.9A.84.E5.B8.B8.E9.87.8F.E8.A1.A8.E8.BE.BE.E5.BC.8F" title="cpp/language/constant expression">可按语境转换到 <code>bool</code> 类型的经转换常量表达式</a>。若其值为 <code>true</code>，则弃用 <span class="t-spar">false分支语句</span>（若存在），否则弃用 <span class="t-spar">true分支语句</span>。
</p><p>弃用语句中的 return 语句不参与函数返回类型推导：
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span> <span class="sy1">&lt;</span><span class="kw1">typename</span> T<span class="sy1">&gt;</span>
<span class="kw4">auto</span> get_value<span class="br0">(</span>T t<span class="br0">)</span> <span class="br0">{</span>
    <span class="kw1">if</span> <span class="kw4">constexpr</span> <span class="br0">(</span><a href="../types/is_pointer.html"><span class="kw463">std::<span class="me2">is_pointer_v</span></span></a><span class="sy1">&lt;</span>T<span class="sy1">&gt;</span><span class="br0">)</span>
        <span class="kw1">return</span> <span class="sy2">*</span>t<span class="sy4">;</span> <span class="co1">// 对 T = int* 推导返回类型为 int</span>
    <span class="kw1">else</span>
        <span class="kw1">return</span> t<span class="sy4">;</span>  <span class="co1">// 对 T = int 推导返回类型为 int</span>
<span class="br0">}</span></pre></div></div>
<p>弃用语句可以 <a href="definition.html#.E5.8D.95.E4.B8.80.E5.AE.9A.E4.B9.89.E8.A7.84.E5.88.99.EF.BC.88ODR.EF.BC.89" title="cpp/language/definition">ODR 式使用</a>未定义的变量
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">extern</span> <span class="kw4">int</span> x<span class="sy4">;</span> <span class="co1">// 不需要 x 的定义</span>
<span class="kw4">int</span> f<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
<span class="kw1">if</span> <span class="kw4">constexpr</span> <span class="br0">(</span><span class="kw2">true</span><span class="br0">)</span>
    <span class="kw1">return</span> <span class="nu0">0</span><span class="sy4">;</span>
<span class="kw1">else</span> <span class="kw1">if</span> <span class="br0">(</span>x<span class="br0">)</span>
    <span class="kw1">return</span> x<span class="sy4">;</span>
<span class="kw1">else</span>
    <span class="kw1">return</span> <span class="sy2">-</span>x<span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<p>若 constexpr if 语句出现于<a href="templates.html#.E6.A8.A1.E6.9D.BF.E5.8C.96.E5.AE.9E.E4.BD.93" title="cpp/language/templates">模板实体</a>内，且若 <span class="t-spar">条件</span> 在实例化后不是<a href="dependent_name.html#.E5.80.BC.E5.BE.85.E5.86.B3.E8.A1.A8.E8.BE.BE.E5.BC.8F" title="cpp/language/dependent name">值待决</a>的，则外围模板被实例化时不会实例化弃用语句。
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">typename</span> T, <span class="kw1">typename</span> ... <span class="me1">Rest</span><span class="sy1">&gt;</span>
<span class="kw4">void</span> g<span class="br0">(</span>T<span class="sy3">&amp;&amp;</span> p, Rest<span class="sy3">&amp;&amp;</span> ...<span class="me1">rs</span><span class="br0">)</span> <span class="br0">{</span>
    <span class="co1">// ... 处理 p</span>
    <span class="kw1">if</span> <span class="kw4">constexpr</span> <span class="br0">(</span>sizeof...<span class="br0">(</span>rs<span class="br0">)</span> <span class="sy1">&gt;</span> <span class="nu0">0</span><span class="br0">)</span>
        g<span class="br0">(</span>rs...<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 始终不会对空实参列表实例化。</span>
<span class="br0">}</span></pre></div></div>
<p>在模板外，弃用语句受到完整的检查。<code>if constexpr</code> 不是 <a href="../preprocessor/conditional.html" title="cpp/preprocessor/conditional"><tt>
#if</tt></a> 预处理指令的替代品：
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
    <span class="kw1">if</span> <span class="kw4">constexpr</span><span class="br0">(</span><span class="kw2">false</span><span class="br0">)</span> <span class="br0">{</span>
        <span class="kw4">int</span> i <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span>
        <span class="kw4">int</span> <span class="sy2">*</span>p <span class="sy1">=</span> i<span class="sy4">;</span> <span class="co1">// 在弃用语句中仍为错误</span>
    <span class="br0">}</span>
<span class="br0">}</span></pre></div></div>
<p>注意：实例化后仍为值待决的一个例子是嵌套模板，例如
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span> <span class="kw4">void</span> g<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
    <span class="kw4">auto</span> lm <span class="sy1">=</span> <span class="br0">[</span><span class="br0">]</span><span class="br0">(</span><span class="kw4">auto</span> p<span class="br0">)</span> <span class="br0">{</span>
        <span class="kw1">if</span> <span class="kw4">constexpr</span> <span class="br0">(</span>sizeof<span class="br0">(</span>T<span class="br0">)</span> <span class="sy1">==</span> <span class="nu0">1</span> <span class="sy3">&amp;&amp;</span> sizeof p <span class="sy1">==</span> <span class="nu0">1</span><span class="br0">)</span> <span class="br0">{</span>
           <span class="co1">// 此条件在 g&lt;T&gt; 实例化后仍为值待决的</span>
        <span class="br0">}</span>
    <span class="br0">}</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<p>注意：弃用语句不能对所有特化均非良构：
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span> <span class="sy1">&lt;</span><span class="kw1">typename</span> T<span class="sy1">&gt;</span>
<span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
     <span class="kw1">if</span> <span class="kw4">constexpr</span> <span class="br0">(</span><a href="../types/is_arithmetic.html"><span class="kw483">std::<span class="me2">is_arithmetic_v</span></span></a><span class="sy1">&lt;</span>T<span class="sy1">&gt;</span><span class="br0">)</span>
         <span class="co1">// ...</span>
     <span class="kw1">else</span>
       static_assert<span class="br0">(</span><span class="kw2">false</span>, <span class="st0">"必须是算术类型"</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 非良构：该语句对于所有 T 都非法</span>
<span class="br0">}</span></pre></div></div>
<p>对这种万应语句的常用变通方案，是一条始终为 false 的类型待决表达式：
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span> <span class="kw1">struct</span> dependent_false <span class="sy4">:</span> <a href="../types/integral_constant.html"><span class="kw650">std::<span class="me2">false_type</span></span></a> <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">template</span> <span class="sy1">&lt;</span><span class="kw1">typename</span> T<span class="sy1">&gt;</span>
<span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
     <span class="kw1">if</span> <span class="kw4">constexpr</span> <span class="br0">(</span><a href="../types/is_arithmetic.html"><span class="kw483">std::<span class="me2">is_arithmetic_v</span></span></a><span class="sy1">&lt;</span>T<span class="sy1">&gt;</span><span class="br0">)</span>
         <span class="co1">// ...</span>
     <span class="kw1">else</span>
       static_assert<span class="br0">(</span>dependent_false<span class="sy1">&lt;</span>T<span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">value</span>, <span class="st0">"必须是算术类型"</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// ok</span>
<span class="br0">}</span></pre></div></div>
<p>在 constexpr if 的子语句中出现的标号（<a href="goto.html" title="cpp/language/goto">goto 目标</a>、<code>case</code> 标号和 <code>default:</code>），只能在同一子语句中（由 <a href="switch.html" title="cpp/language/switch">switch</a> 或 <a href="goto.html" title="cpp/language/goto">goto</a>）引用。
</p>
</td>
<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td></tr>
</table>
<h3><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">注解</span></h3>
<p>若 <span class="t-spar">true分支语句</span> 或 <span class="t-spar">false分支语句</span> 不是复合语句，则按如同是复合语句一样处理：
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">if</span> <span class="br0">(</span>x<span class="br0">)</span>
    <span class="kw4">int</span> i<span class="sy4">;</span>
<span class="co1">// i 不再在作用域中</span></pre></div></div>
<p>与下面相同
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">if</span> <span class="br0">(</span>x<span class="br0">)</span> <span class="br0">{</span>
    <span class="kw4">int</span> i<span class="sy4">;</span>
<span class="br0">}</span> <span class="co1">// i 不再在作用域中</span></pre></div></div>
<p><span class="t-spar">条件</span> 若是声明，则其所引入的名字的作用域是两个语句体的合并作用域：
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">if</span> <span class="br0">(</span><span class="kw4">int</span> x <span class="sy1">=</span> f<span class="br0">(</span><span class="br0">)</span><span class="br0">)</span> <span class="br0">{</span>
    <span class="kw4">int</span> x<span class="sy4">;</span> <span class="co1">// 错误：重复声明了 x</span>
<span class="br0">}</span> <span class="kw1">else</span> <span class="br0">{</span>
    <span class="kw4">int</span> x<span class="sy4">;</span> <span class="co1">// 错误：重复声明了 x</span>
<span class="br0">}</span></pre></div></div>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx14"><td>
<p>若通过 <a href="goto.html" title="cpp/language/goto">goto</a> 或 <span class="t-lc">longjmp</span> 进入 <span class="t-spar">true分支语句</span>，则不执行 <span class="t-spar">false分支语句</span>。
</p>
</td>
<td><span class="t-mark-rev t-since-cxx14">(C++14 起)</span></td></tr>
<tr class="t-rev t-since-cxx17"><td>
<p>不允许 switch 和 goto 跳入 constexpr if 语句的分支。
</p>
</td>
<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td></tr>
</table>
<h3><span class="mw-headline" id=".E5.85.B3.E9.94.AE.E8.AF.8D">关键词</span></h3>
<p><a href="../keywords/if.html" title="cpp/keyword/if"><tt>if</tt></a>,
<a href="../keyword/else.html" title="cpp/keyword/else"><tt>else</tt></a>,
<a href="../keyword/constexpr.html" title="cpp/keyword/constexpr"><tt>constexpr</tt></a>
</p>
<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
<table class="t-dsc-begin">

</table>

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